1. Field of the Invention
The present invention relates generally to the field of antifriction bearings for use in rotating equipment. More particularly, the invention relates to an improved technique for protecting metallic surfaces of antifriction bearings from corrosion, thereby enhancing the useful life of the bearings in a variety of applications and atmospheres.
2. Description of the Related Art
A great number of applications exist for antifriction bearing assemblies in rotating equipment. In general, such bearing assemblies are positioned between rotating and stationary machine elements, and serve to support the rotating machine element on the stationary machine element. Typical industrial applications include shaft bearings, such as in electric motors and pulleys, wheel bearings, such as in roller conveyors and monorails, and so forth. Conventional antifriction bearings generally consist of an inner race, an outer race, and a series of antifriction bearing elements disposed between the inner and outer races. The inner and outer races form hardened raceways on which the bearing elements ride. The bearing elements are free to rotate on the inner and outer races, thereby facilitating rotation of the stationary and rotary elements with respect to one another by virtue of the rolling contact with the bearing races. Such bearing assemblies may include additional components, such as cages, collars, retaining rings, and so forth to facilitate mounting and retention of the bearing in a desired location in the rotating equipment.
Throughout industry, antifriction bearings of the type described above are required to operate reliably in a wide range of environments. In a typical factory environment, components of the surrounding atmosphere may allow conventional materials, such as steel and cast iron to be employed without significant degradation of the bearing components over their useful life. However, in a number of applications, special materials must be provided in the bearing components to avoid accelerated corrosion resulting from the atmosphere within which the bearing operates. For example, in the food and beverage industry, production and transport lines are periodically washed with high pressure steam and water jets, subjecting the bearing components to severe temperature and moisture cycles which can accelerate corrosion of the component surfaces. Similarly, in chemical processing and marine applications, concentrations of corrosive chemicals in the atmosphere surrounding the bearings, such as strong acids, alkalis or salts, can accelerate degradation of the bearing surfaces. In a number of such applications, the corrosion may not only threaten the mechanical integrity of the bearing components, but may end the useful life of the bearing by threatening to contaminate processed goods, such as food and beverage products.
Several approaches have been adopted and are currently in use for protecting bearing components from corrosion. For example, base materials have been used to form bearing components which are less susceptible to corrosive products. Such materials include stainless steel and more exotic metals. Similarly, exposed bearing components have been treated by various plating processes to create corrosion resistant surfaces. Commercially available antifriction bearings of this type include chrome-plated and zinc-nickel plated rings. Depending upon the plating process employed, the plated components may benefit from a simple barrier layer provided between the underlying substrate material and the atmosphere, or may receive additional galvanic protection by virtue of the materials employed in the plating.
While plated bearing products of this type provide enhanced corrosion protection as compared to bare substrate materials used alone, they are not completely satisfactory. For example, certain plating processes are relatively costly considering the cost of the plating material in combination with the labor required for the plating process. Moreover, certain commercially available plating processes may offer mechanical protection without galvanic protection or vice versa. In addition, a large number of applications exist in which visual inspection is the primary means for detecting accelerated bearing corrosion. Accordingly, where the plating provided on the bearing components does not offer a uniform or consistent appearance, or deviates significantly from the appearances of known products, inspection personnel may experience difficulties in evaluating the state of the bearing components. In practice, the appearance of the bearing components is much more than cosmetic, but often forms the basis for evaluating the condition and life of the bearing.
There is a need, therefore, for an improved technique for protecting bearing components from corrosion. In particular, there is a need for a plating or coating for such bearing components which offers both galvanic protection for the underlying substrate material, as well as a mechanical barrier to the surrounding atmosphere. There is currently a significant need for a plated bearing product which offers these advantages over heretofore known bearings, but which conforms in appearance and performance to more conventional materials, such as austenitic stainless steel.